The present application relates to measuring the refractive index of a sample using optical sensing.
Solid state Mach-Zehnder Interferometers (MZI) have been used in measuring the refractive index of a medium, sensing of label-free bio-chemical materials and DNA hybridization. FIG. 1 is a simplified schematic diagram of a MZI-based sensor 10 having an input port 10 and an output port 20, as known in the prior art. The optical signal received at input port 10 is split into two beams via splitter/coupler 35 and caused to travel in optical paths 15 and 20 formed using, for example, silicon waveguides. Optical path 15 includes a window 30 adapted to hold a sample of the material or compound (hereinafter alternatively referred to as sample) whose refractive index, or changes in its refractive index due to a chemical reaction, is being measured. No such window is present in optical path 25. The optical signals travelling in paths 15 and 25 are combined by splitter/coupler 40 and supplied at output port 20. If the optical signals arriving at splitter/coupler 40 have the same phase, they add constructively to form a maxima. Conversely, if the optical signals arriving at splitter/coupler 40, 20 are 180° out-of-phase, they thus cancel out each other. Other relative phase differences result in other values of the optical signal power at output port 20 of MZI-based sensor 10.
The difference between the phases of the two optical signals arriving at splitter/coupler 40—and therefore the power of the optical signal at output port (hereinafter alternatively referred to as the output signal) 20—is dependent on the refractive index of the sample disposed in window 30 as well as the length L of window 30.
In order to increase the sensitivity of an MZI-based sensor (alternatively referred to herein as sensor), the dependency of the effective refractive index of the waveguide to the sample's refractive index needs to increase. To achieve this, in some conventional MZI-based sensors, the ratio of the evanescent component of the electromagnetic (EM) field to the propagating component of the EM field is increased by either reducing the size of the waveguide core, or reducing the refractive index contrast of the waveguide to that of the material enclosing the waveguide. In silicon photonics, the high refractive index of a Silicon waveguide relative to the refractive index of the medium, such as Silicon dioxide, enclosing the waveguide results in a small ratio of the evanescent field component relative to the propagating field component, thereby causing the sensitivity of the sensor for a given exposure window length to decrease.
Another shortcoming of a conventional MZI-based sensor is the inverse relationship between its sensitivity and the range of refractive indices it is able to sense. This is due to the fact that the response of a conventional MZI-based sensor to two optical signals that have a phase difference of φ and (2mπ+φ), where in is an integer, is the same. Accordingly, unless the approximate refractive index of the sample is known to within a narrow range, conventional MZI-based sensors are unable to uniquely determine a sample's refractive index.
FIG. 2 is a simplified schematic diagram of an MZI-based sensor 200, as is also known in the prior art. MZI-based sensor 200 includes an input port 210 and a pair of output ports 280 and 290. The optical signal received at input port 210 is split into two beams travelling in optical waveguides (paths) 215 and 225. Optical waveguide 215 is split into two equal paths 230 and 235 at splitter/coupler 218, and optical waveguide 225 is split into two equal paths 240 and 245 at splitter/coupler 338. Optical waveguides 230 and 235 are combined by splitter/coupler 228 leading to an output port 228. Likewise, optical waveguides 240 and 245 are combined by splitter/coupler 248 leading to an output port 248. Waveguides 230 and 240 respectively include exposure windows 260 and 270 adapted to include a sample whose index of refraction is being measured. Window 260 is longer and is thus more sensitive than window 270 but has a smaller one-to-one index range than window 270.
Since exposure windows 260 and 270 are spatially separated, the samples dispensed therein may not be sensed similarly if, for example, there is a gradient in the index of refraction of the sample solution. Also, the two exposure windows 200 and 270 may provide different chemical reaction rates.